* Implement /_synapse/admin/v1/register
This is implemented identically to Synapse, so scripts which work
with Synapse should work with Dendrite.
```
Test 27 POST /_synapse/admin/v1/register with shared secret... OK
Test 28 POST /_synapse/admin/v1/register admin with shared secret... OK
Test 29 POST /_synapse/admin/v1/register with shared secret downcases capitals... OK
Test 30 POST /_synapse/admin/v1/register with shared secret disallows symbols... OK
```
Sytest however has `implementation_specific => "synapse"` which stops these
tests from running.
* Add missing muxes to gobind
* Linting
